1. Field of the Invention
The present invention relates to an original size detecting apparatus, an original size detecting method, and a program for original size detection, which are applied e.g. to a digital copying machine, a scanner, and a facsimile machine, for detecting the size of an original subjected to image reading.
2. Description of the Related Art
Conventionally, various types of original size detecting apparatuses for detecting the size of an original subjected to image reading have been proposed. For example, there has been proposed an original size detecting apparatus of a type in which the leading end of an original is irradiated by a light source, reflected light from the original is read by a CCD (charge coupled device), and at the same time the dimension of the original in the sub-scanning direction is detected by an original size sensor, whereafter the size of the original is detected based on the result of the reading by the CCD and that of the detection by the original size sensor (see Japanese Laid-Open Patent Publication (Kokai) No. H05-207239).
In the above-mentioned type of original size detecting apparatus, the width of an original in the main-scanning direction is detected using the CCD for use in original reading, the number of original size sensors can be reduced, which enables low-cost and efficient detection of original size.
Further, there has been proposed an original size detecting apparatus of another type which detects the size of an original by sensing opening/closing of an original presser plate that presses an original subjected to image reading against the upper surface of an original platen glass as well as an open state of the original presser plate (see Japanese Laid-Open Patent Publication (Kokai) No. H09-329848).
Original size detection is basically possible by either one of the above-mentioned types. However, when reflected light from an original is read, external scattered light, such as indoor light, from outside the original size detecting apparatus is also read simultaneously. As a result, the reflected light from the original cannot be distinguished from the external scattered light, which can cause erroneous detection of the size of the original.
To prevent the influence of external scattered light, there has been proposed an original size detecting apparatus of a type which detects the original size when an original presser plate is closed. In this type, for example, the color density of the underside surface of the original presser plate is reduced so as to enable detection of a white original (see Japanese Patent Publication (Kokoku) No. S62-47026).
However, in the above-mentioned type in which the color density of the underside surface of the original presser plate is reduced so as to enable detection of a white original, original size detection is possible, but there occurs a “show-through” phenomenon that the background of the original is darkened at the time of reading the original. This “show-through” phenomenon is conspicuous in reading an original with high transmittance, such as a mother print, which causes inconvenience in reading an original.
To prevent the influence of external scattered light, there has been also proposed another type of original size detecting apparatus in which after opening/closing of an original presser plate that presses an original subjected to image reading is detected, distribution of external scattered light detected when the original presser plate is open is stored, and the surface of the original is scanned with a light source turned on when the original presser plate is closed, and then the stored distribution of external scattered light is subtracted from a reflected light signal outputted from a photoelectric conversion means having received the reflected light, to thereby determine the size of the original (see Japanese Laid-Open Patent Publication (Kokai) No. H09-135330).
However, even this type can cause erroneous detection of the size of the original, when the amount of light irradiated from the light source used to scan the original surface is decreased for energy saving purposes while increasing the sensitivity of the photoelectric conversion means so that external scattered light can be detected with very high sensitivity, or when an original subjected to reading is formed of a sheet having a high transmittance, such as a mother print, the photoelectric conversion means senses the influence of the external scattered light.